Worldwide interest in light-emitting diode (LED) technology has rapidly increased over the past two decades. Starting with inorganic LEDs developed in the 60s, they have found their way into numerous lighting, signaling, and display applications, such as, automotive lighting, architectural lighting, flashlights, and backlights for LCD-based displays. Since the turn of the century they have started to appear in more mainstream lighting applications, which as a result of their long life and very high efficacy will result in significant savings in energy usage. This set of applications include traffic signaling lights, street lights, and most recently, residential lighting.
Organic-based LEDs (OLED) were developed in the late 70s (Tang et al, Appl. Phys. Lett. 51, 913 (1987)) and have just recently begun to appear in commercial display applications, such as, televisions, picture frames, and digital camera displays. In the last 5 years or so, good progress has also been made to make OLEDs a viable option for general lighting applications. Despite large gains in their efficiency, OLED lighting will likely remain a niche application due to their environmental sensitivity, shorter lifetime, and low output power density. The latter issue is the dominant one since it requires OLED lighting products to have large surface areas in order to produce acceptable amounts of lumens.
In spite of the deepening penetration of inorganic LEDs into mainstream lighting, unresolved issues still remain, such as, high cost, poor color, and sub-desirable efficiency. Overall there are two ways for creating white LEDs (M. Krames et al., J. Display Technol. 3, 160 (2007)), combining blue, green, and red LEDs to form white LED arrays or combining a blue LED with appropriate down conversion phosphors to create a white light source. The first way yields a higher overall efficiency. Despite very high internal quantum efficiencies for red and blue LEDs of approximately 90% and 70%, respectively, the IQE of green LEDs at the desirable wavelengths of 540-560 nm is below 10%. This “green gap” issue has been recognized for many years (large strain develops in the active region as a result of incorporating sufficient In in the GaN in order to form green emitting InGaN) and despite numerous efforts, still remains largely unresolved. Combining blue GaN LEDs with appropriate phosphors has recently yielded white LEDs with efficacies over 120 Lumens/Watt. Unfortunately, the correlated color temperature (CCT) of the corresponding white is typically high (>6000 K), yielding a cold light which lacks sufficient red response. Another outstanding issue is the efficiency of the phosphors, which for commercial phosphors are currently at 65% (include the efficiency hit due to the Stoke shift) (D. Haranath et al., Appl. Phys. Lett. 89, 173118 (2006)). Both inorganic LED approaches for white light, as of today, are approximately a factor of 100 too costly to engender significant market penetration into the residential market without significant government subsidies or incentives.
As discussed above, despite the impressive efficiency and large penetration to date of inorganic LEDs (to be called LEDs) into lighting applications, outstanding issues still remain. Focusing on color-mixed LEDs (combining red, green, and blue LEDs), the two pressing issues are high cost and the sub-par performance of green LEDs. A large part of the high cost is associated with conventional LEDs being grown on crystalline substrates. More specifically, sapphire or SiC for blue and green LEDs and GaAs for red LEDs. As discussed above, the sticking point associated with creating efficient InGaN-based LEDs is that incorporating In into the active region results in significant strain relative to the cladding layers (W. Lee et al., J. Display Technol. 3, 126 (2007)).
Recently, there has been significant research activity towards creating nanowire-based LEDs, where the nanowires are grown using MOVPE techniques by either a template (S. Hersee et al., Electron. Lett. 45, 75 (2009)) or vapor liquid solid (VLS) approach (S. Lee et al. Philosophical Magazine 87, 2105 (2007)). The advantages of employing nanowires as LED elements are that they can be grown on inexpensive substrates (such as glass) and the amount of lattice mismatch that can be tolerated between LED layers is significantly higher when the crystalline material is a 20-100 nm thick nanowire as compared to bulk heterostructure growth (D. Zubia et al., J. Appl. Phys. 85, 6492 (1999)). Green LEDs can be formed by two ways, incorporating InGaN emissive layers in GaN-based pin nanowires, or by forming II-VI material based pin nanowires. Progress has been made on both fronts, but many issues still remain unresolved. For GaN-based nanowires, efficient doping is still problematic and the quantum efficiency of the emitters remains sub-par (S. Hersee et al., Nano Lett. 6, 1808 (2006)). For II-VI material based pin nanowires, green LEDs can be formed by employing CdZnSe or ZnSeTe in the active region; however, the number of unresolved issues is even larger.
Progress in creating highly emissive (C. Barrelet et al., JACS 125, 11498 (2003)) and dopable II-VI nanowires has been limited. Almost no mention has been made of successful doping of II-VI nanowires, or where doping has been mentioned it is stated that the undoped ZnSe nanowires have low resistivity, ˜1 ohm-cm (J. Salfi et al. Appl. Phys. Lett. 89, 261112 (2006)), which implies a high degree of defects since undoped ZnSe should be highly resistive (>105 ohm-cm). With regard to emissive characteristics, the photoluminescence (PL) of high quality epi-material should show band gap exciton features and a very small amount of mid-gap defect emission. All reported ZnSe nanowires show large levels of defect emission in their PL response (X. Zhang et al., J. Appl. Phys. 95, 5752 (2004)). The one article (U. Philipose et al., J. Appl. Phys. 100, 084316 (2006)) in which the defect emission was reduced was where added Zn was post-growth diffused into the ZnSe nanowires in order to reduce the large amount of Zn vacancies present after nanowire growth. Performing an extra diffusion step is costly and unworkable when the emitter layer is part of a pin diode device structure. Consequently, in spite of the technological importance of device quality II-VI nanowires, problems remain.